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Holy Atoll!

[Excerpt from blog entry May 30, 2010 – Aboard IO, a two year Pacific crossing]

Position, 16 30.3S 145 27.3W

We are at the southern end of Fakarava and today we swam with over 50 sharks! Three species: gray, white tip and black tip sharks were everywhere.
We started off the day with a brisk sail south (beating to windward, of course), to the south end of the atoll, dropped the hook in 45 feet of crystal clear water near our friends on Mulan and Oso Blanco. Within one hour we were all surrounded by friendly reef sharks and a myriad of other tropical reef fish having the best snorkel of our lives!
The atolls are basically a big ring of coral reef and occasionally there is a passage-way that is deep enough that you can enter with a boat. There is a trick though, in that you can only enter at certain times due to the very strong currents that flow through these passes. When the tide rises, the water outside the atoll rises and to equilibrate the water level, water must flow in. The reverse occurs during low tide and you get a strong outflow current happening. Depending on the size of the atoll, and since the channels are narrow (some are only 30 meters wide), the currents can exceed 4-6 knots. Given these conditions, entering them can be like entering a fast flowing river complete with standing waves, current eddies and whirlpools.
These passes are biologically relevant in that they also allow nutrient-rich water from the outside to flow into the relatively nutrient-poor lagoon. Like anywhere, where the food is, the life is in abundance.
Once we arrived and IO was safely anchored, we all piled on to Oso Blanco’s tender, drove to the entrance of the channel, and jumped overboard in 50 feet of water. The water clarity allowed us to see the bottom and we were instantly surrounded by a rainbow of color and the very dramatic and abundant presence of the sleek chondrichthyes (sharks). Once in the water, we just sat back and let the 3 knot inward bound current push us (rather rapidly) into the atoll lagoon. It was like a brilliant movie being played out before our eyes, so you just sat back and let it all go by! Once we had been pushed sufficiently far inside the lagoon, our boat tender picked us up and took us back to the start and we jumped in and did it all over again. It was absolutely the most brilliant snorkel I have ever done!
After that fun, we returned to IO and I started playing with the local wildlife around the boat – heaps of surgeon fish and more black-tip reef sharks. I had heard a story from a local fisherman that the sharks here have been conditioned to the sound of a spear-gun firing. This fellow used to hunt for lagoon fish with speargun and said that once you have speared a fish, you basically have 45 seconds to get the fish out of the water before the sharks show up and take it away from you. So, due to the abundance of spear fishing done, the sharks have learned to associate the sound of a speargun with a free meal. Well, that sounds like an easily testable hypothesis, so I jumped in the water and when there were no sharks within view, and fired my speargun. Now, when a speargun is fired off underwater it makes a very distinct high-pitched metallic report that can be heard a long distance off (even by humans). Within 30 seconds of firing my gun, I had a 1.5 meter (5 foot) black-tip shark less than 5 meters from me! In about a minute and a half, there were 5 sharks and all of them were clearly demonstrating searching behavior, basically looking for that free meal. Fantastic! But admittedly, they were clearly being a little more aggressive so I chose to hastily exit the water.